1. Field of the Invention
This invention relates to brake boosters for vehicles and more particularly to a tandem diaphragm brake booster of the type used in association with a vehicle brake system.
2. Description of the Prior Art
As shown in FIG. 4, one of the typical tandem diaphragm brake boosters includes a housing 100, first and second diaphragms 110, 120 disposed in said housing 100, a partition wall 130 disposed between said first and second diaphragms 110, 120 within said housing 100 for defining first constant pressure chamber 140, first variable pressure chamber 150, second constant pressure chamber 160 and second variable pressure chamber 170, a piston member 180 secured to said first and second diaphragms 110, 120, a valve member 190 disposed in the piston member 180 for generating a pressure differential between the constant pressure chambers 140, 160 and the variable pressure chambers 150, 170 and operator-operated means 195 for actuating said valve member 190 in response to manual operation.
The conventional tandem diaphragm brake boosters are classified into two types in relation to the passage connecting the first variable pressure chamber 150 and second variable pressure chamber 170.
One type of the brake booster has the passage formed within the piston member. This type of the brake booster is described in Japanese published unexamined utility model application No. SHO 57-3882. Usually the piston member is very complicated. Therefore, it is not easy to form the passage within the piston member. The other type of the brake booster is shown in U.S. Pat. No. 4,257,312 and also in the above mentioned Japanese published unexamined utility model application. The brake booster shown in FIG. 4 is one. This type of brake booster has the passage 105 formed between the inner surface of the housing 100 and the cylindrical outer periphery of the partition wall 130. It is relatively easy to make the passage 105. However, this type of brake booster has the following drawbacks. As shown in FIG. 4, a front shell section 101 and a rear shell section 102, which together form the housing 100, are joined together air-tightly with a bead 110a of the first diaphragm 110. Namely, the rear shell section 102 should be shaped in a cup with the cylindrical outer portion so that its volume is increased. Usually the rear shell section 102 is made of a thicker steel plate than the front shell section 101. The resulting brake booster becomes heavy. Further, in winter, when used at a cold area where an anti-freeze agent is spread over the roads, this agent splashes up over the engine room during running of the car, and the connecting part of both shell sections 101,102 receive said splashing agent actively, so it is feared that rusting and corrosion will occur in said connecting parts due to accumulation of salt components of said agent.